Cell cycle regulation in normal, non-immortalized cells, e.g., human or mouse fibroblasts, remains poorly investigated. Recent evidence suggests that the progression of such cells into a post-mitotic state ("senescence") or to the alternative fate, immortalization, may reflect a drift in differentiation state which is responsive to gene products that act on higher order chromatin structure. To investigate this possibility, two histone deacetylase genes (one of which was isolated by this group) have been introduced into human fibroblasts using a newly developed retroviral vector system. In related work, genes encoding two histone acetyltransferases, hGCN5 and hP/CAF, were observed to inhibit G1 to S phase transit in HeLa cells. These histone acetyltransferases are likewise being introduced by retrovirus-mediated transduction into human fibroblasts. To examine whether alterations in higher order chromatin structure accompany cell senescence or immortalization, a new method was developed to map euchromatin vs. heterochromatin domains. Application of this technique in the context of "genomic differential display" revealed for the first time loci which switch chromatin state in association with human fibroblast senescence.